Asp.Net Nested Master Page Error

I was working on a website today using nested master pages and suddenly started getting the following error:

'asp' is an unrecognized tag prefix or device filter

It was very odd. One minute the page was fine, the next it was broke. I tried to restart Visual Studio, but still the same error.

After searching around, I finally found this article on MSDN that talks about the error when using nested master pages. Basically if you are using nested master pages, the master pages must also be open at the same time. Weird, but it fixed the problem.

Just something I thought I would pass along.